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The invention relates to a method for manufacturing PTFE preform parts and finished articles. More particularly this invention relates to compression molding of granular PTFE followed by sintering to produce articles of a size and length not possible utilizing conventional PTFE molds or presses.
PTFE (polytetrafluoroethylene) is commonly used for industrial applications such as gaskets, fittings, valve bodies, diaphragms and other applications where a non-reactive or low friction surface is needed. It has a wide range of operating temperatures and is extremely chemical resistant.
Small PTFE parts are manufactured through a compression molding process by disposing granular PTFE in a mold and compressing it under pressure to fuse the particles together. A limitation to this process is the size requirements of the press. The presses are quite expensive and generally larger presses are even more expensive. In typical PTFE compaction a 3-1 compaction ratio is necessary to achieve the desired product. For small parts, such as a 12 inch long tube, a three foot press with a 5 foot clearance will suffice. However, if the desired part is 60 inches in length, a 15 foot press with a 25 foot clearance would be necessary. While 60 inch and larger parts are often needed, presses of this size are not readily available, if at all.
One method to circumvent the need for an unmanageably large press is to manufacture the large part in pieces and thermally fuse the pieces together. Unfortunately, this cannot be accomplished with standard PTFE. Standard PTFE cannot be thermally fused with any degree of success. The finished product would be brittle at the joints and would break apart easily.
A modified PTFE which is susceptible to thermal fusing could be used instead of conventional PTFE. In this process, individual pieces of modified PTFE are compression molded and then sintered together. However, modified PTFE will become more lubricious in the oven during sintering and become distorted and unable to finish to the dimensions required. Furthermore, a large article comprised of modified PTFE would not be able to hold up to its own weight during sintering and may collapse. Thus, a part comprised of modified PTFE will not maintain its shape in the oven, while an unmodified PTFE part will not fully bond and be brittle at the seams. Using either method has proven unpractical for manufacturing large seamless articles out of PTFE.
Therefore, it would be beneficial to have a method of manufacturing a PTFE containing part which can be made in segments and thermally fused together without losing desirable properties of the PTFE.
A first embodiment of the method of the present invention comprises, disposing a first layer of a modified PTFE resin into a mold cavity, exposing the first layer to a first pressure, disposing a second layer of an unmodified PTFE resin into the mold cavity, and exposing the first and second layers to a second pressure, such that the second pressure is higher than the first pressure.
In another embodiment of the present invention the method further comprises disposing a third layer of modified PTFE resin in the mold cavity and applying a third pressure wherein the third pressure is higher than the first pressure or the second pressure. Large articles may be formed by manufacturing more than one preform, assembling the preforms so that the first modified layer of one preform contacts the first or third modified layer of another platform, and sintering the assembly.
In a further embodiment of the present invention, a method for manufacturing a PTFE article is presented comprising, manufacturing a PTFE preform by disposing a first layer of a modified PTFE resin into a mold cavity, exposing the first layer to a first pressure, disposing a second layer of an unmodified PTFE resin into the mold cavity, exposing the first and second layers to a second pressure, optionally disposing a third layer of modified PTFE resin into the mold cavity and exposing the first, second and third layers to a third pressure. Then placing the preform parts adjacent to one another such that the layer of modified resin of one part is contacting the layer of modified resin of the other part, and sintering the parts together form a single article.
A feature and advantage of the present invention provides a method of manufacturing articles of a size, length and shape that heretofore was not possible due to machinery limitations.
Another feature and advantage of the present invention is a method for producing large PTFE articles which are strong and seamless.
Another feature and advantage of the present invention is a method for bonding PTFE pieces without creating weak or brittle seams or joints.
Another feature and advantage of the present invention is a method for sintering PTFE pieces without loosing their shape during the sintering process.
Another feature and advantage of the present invention is a method for manufacturing large, net or near net PTFE articles.
These and other aspects of the subject invention and the method of using the same will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings described herein below.